Watching the birth of a nanocrystal superlattice

Two-dimensional systems have recently emerged as very promising candidates for future electronic devices. An ordered 2D array of nanocrystals can be created by letting nanocrystals self-assemble and connect at a liquid surface. This new structure shows excellent long range order on both the nanocrystal and atomic level. Experiments at beamline ID10 revealed how the nanocrystals order themselves and ‘click together’ to produce beautiful 2D superlattices.

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PbSe nanocrystals self-assemble at the liquid/air interface. We have studied this phenomenon using multiple techniques: small-angle X-ray scattering (SAXS) - to study the motion of the nanocrystals on the liquid surface - and wide-angle X-ray scattering (WAXS) - to study the orientation and self-assembly of the nanocrystals at the interface. We wanted to perform these experiments in grazing incidence mode (GI), which means that the X-ray beam impinges at a very tiny angle of only 0.3° with respect to the liquid/air interface. This made ID10 the beamline of choice as it is specialised in grazing-incidence scattering experiments on liquid surfaces.

We followed the formation of nanocrystals on two different length scales: at the nanoscale for the movement of nanocrystals on the liquid surface with GISAXS; and at the atomic length scale with GIWAXS, which probes both the orientation of the nanocrystals with respect to the liquid surface and the crystalline domain size. The results of the in situ X-ray scattering data, together with ex situ electron microscopy and diffraction, are shown in Figure 1.

Figure 1. The different stages during the self-assembly process as monitored by ex situ electron microscopy and electron diffraction and in situ GISAXS/GIWAXS.

By combining all the data and performing Monte Carlo simulations, a detailed picture of the self-assembly process was obtained, which is presented in Figure 2. The nanocrystals are initially dispersed in an apolar solvent, which is placed onto a polar liquid substrate. These two solvents do not mix, and the nanocrystals are not soluble in the bottom liquid. At stage 1 of the self-assembly process, the apolar solvent of the nanocrystal solution gradually evaporates and the increasing concentration of nanocrystals forces them to slowly attach to the interface. At stage 2, most of the apolar solvent in which the nanocrystals were dissolved has evaporated, and the nanocrystals self-assemble into an energetically favored dense hexagonal array of nanocrystals. During stage 3, this hexagonal monolayer starts to deform towards the final square lattice. At stage 4, the nanocrystals are in close proximity and can atomically fuse together by forming crystalline bridges between the nanocrystals.

Figure 2. Model for the self-assembly, acquired from a combination of the in situ X-ray scattering data, the ex situ electron microscopy and Monte Carlo simulations.

Synchrotron radiation played a pivotal role in this study. While electron microscopy provided important insights into the structures formed at different stages of the process, it is essentially ex situ. Intermediate structures formed at stage 2 were only observed using synchrotron techniques as they do not survive upon sample drying on a grid. Further insights into the mechanism were provided by the computer simulations that indicate that there must be an attraction between the nanocrystal {100} facets, which are caused by electrostatic or strong Van der Waals interactions.

We have succeeded for the first time in capturing this incredible sequence of phase transitions of nanocrystals during their self-assembly process on a liquid interface. The insights obtained may result in bottom-up routes towards a diversity of 2D electronic or photonic materials based on nanocrystals. The presented procedures will be developed further, such that the self-assembly of nanocrystal colloids can become a feasible alternative to top-down lithography based methods.